Temperature sensor

ABSTRACT

The disclosure herein relates to a method for monitoring the temperature of a material transparent to a given part of the electromagnetic spectrum or of a body in good thermal contact with the material. The preferred method referenced herein utilizes laser radiation to measure variations in the index of refraction of a material with temperature variations. Alternative embodiments refer to variations in the transmission coefficient and reflection coefficient with variation in temperature.

ited States Patent Weil [451 June 27, 1972 [541 TEMPERATURE SENSOR3,453,434 7/1969 Takami et al.. .....73/355 x [72] Inventor: Rmul Brweuolivene M0 3,462,224 8/1969 Woods et a1. ..73/355 UX [73] Assignee:Monsanto Company, St. Louis, Mo. Primary Examiner-Louis R. PrinceAssistant Examiner-Frederick Shoon 22 F1 d: 19 9 1 18 Dec l 69Attarney-William I. Andress, John D. Upham and Neal E. [21] Appl. N0.:886,637 Willis 52 0.8. CI. ..73/339 R, 73/355 R, 250/833 H, [57]ABSTRACT 350/160 P The disclosure herein relates to a method formonitoring the [51] Int. Cl. 11/12 temperaxure f a t i l transparent t agiven n f th [58] Field of Search ..73/339, 355; 250/833 IR;electromagnetic Spectrum or of a body in good thermal Com 350/160 160 Ptact with the material. The preferred method referenced 56 R f ed hereinutilizes laser radiation to measure variations in the e erences It indexof refraction of a material with temperature variations.

UNITED STATES PATENTS Alternative embodiments refer to variations in thetransmission coefficient and reflection coefficient with variation 1n3,591,810 7/1971 Jackson ..350/l60 P temperature 2,792,484 5/1957Gurewitsch et al. ..73/355 X 3,252,374 5/1966 Stookey ..350/1 60 P 5Claims, 6 Drawing Figures IRISES I 4 5 CHOPPER CO2 LASER DETECTOR I II'I l [5 l l 1 I I4 1 '3 1 1 J I l X-Y RECORDER 1 I, AMPLIFIER l I I6 I fi 1 STRIP-CHART f RECORDER J PATENTEDJUIIZ? I972 SHEET 2 [IF 3 25.5 260TEMPERATURE C FIG. 2.

[8 TEMPERATURE DETECTOR INDICATORZO REFLECTOR SIGNAL PROCESSOR DETECTORGo As MOPULATOR REFLECTOR e0 LASER TEMPERATURE SENSITIVITY RANGE(CIDETECTOR ENERGY SOURCE FIG. 4.

PATENTEDJUII 27 Ian SHEET 30F 3 26 SENSOR MATERIAL REFLECTIVE COATINGFIG. 5.

CONTACT BODY REFLECTIVE COATING B REFLECTOR 29 CO2 LAsER DETECTORDETECTOR SIGNAL PROCESSOR TEMPERATURE INDICATOR FIG. 6.

TEIVWERATURE SENSOR BACKGROUND OF THE INVENTION This invention relatesto the field of temperature measurement.

Current and prior art methods commonly used for measuring temperaturechanges include thermometers utilizing variations in length, volume,pressure, resistivity or magnetic susceptibility with temperature. Othertemperature gauges include thermo-couples and pyrometers using blackbody color as the measure of temperature.

Thermometers now in use have numerous limitations with respect totemperature variation ranges, the inability to precisely detect andmeasure temperature changes at remote distances and in many cases theneed for observing and/or recording temperature changes at the locationof the temperature sensor. In addition, the ability to detect narrow orsmall temperature variations or extreme temperatures is dependent uponthe use of expensive and sometimes sophisticated components, equipmentand apparatus.

SUMMARY OF THE INVENTION The present invention relates to a method forthe precise determination of temperatures and temperature variations ofmaterials over a range of from near absolute zero to close to themelting point of the material. Broadly, the method described hereinmakes use of electromagnetic energy, e.g., laser beams, to measurevariations with temperature in the index of refraction of materialstransparent to a given part of the electromagnetic spectrum to monitorthe temperature and temperature variations of the material or of a bodyin good thermal contact with it. In one embodiment of the inventiontemperature variations are ascertained by making use of variations inthe transmission coefficient with variations in the index of refractionof the material. In another embodiment temperature variations areascertained by making use of the variation in the reflection coefficientof the material when the index of refraction varies. 7

It is, therefore, an object and advantage of the present invention toprovide temperature sensors which do not have the above-mentioned andother limitations of prior art temperature sensors.

BRIEF DESCRIPTION OF THE DRAWINGS In FIG. 1 is shown a schematic diagramof the laboratory apparatus used for' measuring the transmission ofenergy through a sensor material as a function of temperature, and formeasuring the absorption of the material at a given wavelength of inputenergy. I

FIG. 2 is a graph of the energy intensity transmitted through sensormaterial as a function of the material temperature.

In FIG. 3 is shown a schematic diagram of a system configuration fordetecting and measuring the intensity of electromagnetic energytransmitted through a sensor material remote from the energy source andmeans for converting the intensity signal to temperature values.

FIG. 4 is a schematic chart illustrating the relationship between energytransmission through sensor material of differing lengths andtemperature sensitivity ranges (maximumto-minimum transmission)corresponding to said lengths at a given wavelength.

FIG. 5 is a schematic diagram of a system configuration for detectingand measuring the intensity of reflected energy in a sensor material asa function of temperature.

FIG. 6 is a schematic diagram of an application of the systemconfiguration shown in FIG. 5 for detecting and measuring temperaturevariations in a body remote from the energy source.

DESCRIPTION OF PREFERRED EMBODIMENTS The present invention is based uponthe use of variations in the index of refraction of materials to detectand measure tom perature changes of the material or a body in goodthermal contact with the material. The sensor material to be utilized inpracticing this invention must be transparent to the energy source of agiven part of the electromagnetic spectrum and whose length and index ofrefraction change with a change in temperature. These properties permitthe use of energy transmitted through or reflected within the sensor todetect and measure variations in the transmission coefficient andreflection coeflicient when the index of refraction varies in responseto temperature changes.

In the preferred embodiments described herein and of particular utility,the electromagnetic energy source utilized is monochromatic andcoherent, i.e., laser beams, and the sensor material is galliumarsenide, GaAs.

EXAMPLE 1 In one embodiment of the invention, use is made of thevariation in the transmission coefi'lcient of the sensor material with avariation in the index of refraction to detect and measure temperaturechanges. In order to confirm calculated results with measured results,the experimental set-up shown schematically in FIG. 1 was used.

Referring to FIG. 1, the sensor material 4 was a GaAs laser modulatorcrystal (Monsanto MM40) which is a rectangular parallelepiped bar ofsemi-insulating, single crystal, n-type GaAs doped with chromium andhaving a resistivity of 3.0 X 10 ohm-cm and a carrier mobility of 450smlvolt-sec. The GaAs bar was 5.09 10.01 cm long in the l 10] directionwith a cross section of0.3027 X 0.3022 cm in the and [001] directions,respectively. The [1T0] faces were polished flat to within onewavelength of sodium light and parallel to better than 0.003 mm.

The temperature of the GaAs sensor was measured with a Cu-constantanthermocouple 10. This device was made of 0.003 in. wire (OmegaEngineering, Inc.). An NBS calibration (Shenker et a1. Reference TablesFor Thennocouples, NBS Circ. 56l,l955) was used to convert the measuredemfs to temperatures. The calibration was also verified, for thethermocouple used, against an NBS calibrated Pt resistance thermometer,and found to agree within:

Pt Cu Const. =(0.06 0.002 C., over the over the range used. Thethermocouple was fastened to the sensor by means of a 2 X 4 mm strip ofScotch Magic Tape".

. The sensor was held in a groove in a wooden block and covered with 9mm of Styrofoam for insulation (not shown). Heating of the sensor wasaccomplished by shining a 250 W infrared heater lamp on the sensorholder.

The source of radiation 1 was a Seed Electronics Type ML 1C CO laser,operated at about 3 watts CW output. The beam cross section was reducedby means of irises 2 and 3 ahead of the sensor. The diameter of the beamgoing through the sample was 2.0 mm and carried about 0.5 watts of10.6;1. wavelength linearly polarized light. After passing through thesample, the light was chopped by chopper 6 at 13 cps so as to be insynchronism with a tuned amplifier 13 connected to the thermocouple typedetector 9 (Perkin Elmer Type 127-1978). The beam was then stronglyattenuated between the chopper and the detector by attenuators 7 and 8to insure that the detector would operate in its linear region.Attenuator 7 (a partly crossed analyzer) was a Brewster angle germaniumanalyzer set at 84.5 from the direction of polarization of the incidentbeam; the results obtained with this attenuator did not difi'er fromthose obtained with screen-type attenuators.

Attenuator 8 was neutral density screen attenuator. Care was exercisedin positioning the sensor in the beam so as to make the angle ofincidence normal. This insured the space coincidence of the beamsreflected from the two interfaces.

To perform the measurement of transmission as a function of temperature,the output of the detector was directed through preamplifier l2 andamplifier 13 and the amplified signal applied to the Y-axis of an X-Yrecorder 14, or optionally to one pen of a double pen strip-chartrecorder 16. The output of the sensor thermocouple was applied,respectively, through amplifier 15 to the X-axis of the XY recorder oroptionally directly to the other pen of the strip chart instrument. Thecalibration of the instruments used was verified to ascertain theaccuracy of the results. 1

To measure the absorption of the material at the 10.6;1. wavelengthused, another MM 40 modulator made of the same boule, withantireflection coatings, was used. Its temperature rise due toabsorption losses only was measured while the output beam power wassimultaneously measured. The latter measurement was performed bymonitoring with thermocouple R1 the temperature of a calorimeter 5 inwhich the beam was fully absorbed. The calorimeter consisted of a copperblock with a blackened conical cavity into which the beam was directed.The sensor and calorimeter thermocouples outputs were simultaneouslyrecorded on the double pen strip chart recorder, and from these databoth the beam power and the power lost in the GaAs sensor werecalculated. The calorimeter was used onlyfor beam power determination.

FIG. 2 is a graph of the beam intensity (with noise removed from theexperimental curve) going through the sensor as a function of the sensortemperature. The solid line is the measured curve of the recorder traceand the dashed line is the curve calculated by use of the equation belowgiven by Born and Wolf. (Born et al. Principles of Optics, page 628, 2ndEd. 1964. The Macmillan Co., New York. The factor exp(2v n) in thenumerator of Born et al.s Eq. (24) should not have been included and wasnot used in the calculations here. The equation expresses thetransmission of radiation from a medium of index of refraction n througha thin film with index n, and absorption V to a medium n;, In thepresent case the GaAs sensor was treated as the thin film" with index n(rather than n for simplicity in the equations below), and since media 1and 3 were air, n n 1. In the present case of GaAs sensors theabsorption at 10.6 is very small; then, for normal incidence theequation in question reduces to:

where T is the transmission coefficient, n and a are, respectively, theindex of refraction and absorption coefficients of GaAs, both measuredat a temperature to and at the wavelength of the incoming radiation, A Iis the length of the sensor bar and A, is a wavelength of the incidentradiation in air. To calculate the curve, a was taken as 0.012 120.002cm which is the value measured calorimetrically; and n was taken fromthe literature as 3.4 $0.15 (M. Cardona, Proc. Int. Conf. Semicond.Phys., Prague 1960, Academic Press, N. Y. (1961) page 318). The errormarks shown on the graph in FIG. 2 refer to the calculated curve andresult from the limits on the accuracy of the absorption coefficient andindex of refraction (the probable error is higher in the valleys of thecurves than at the peaks).

The oscillations of the curve, i.e., the variation in transmittance withtemperature I, arise almost entirely from changes in the cosine term inEquation l If one lets 1 /M)(7+ where -y is the expansion coefficient(l/I) (dlldt), and 6 is the temperature coefficient of the refractiveindex, (l/n) dn/dt), and t stands for temperature.

In Equation (3), a complete cycle will have been traversed for Arl1= 21r;-ifl n, A, and 'y are known, and the A r corresponding to Ail: ismeasured, one can determine From the measurement of the temperaturedifierence in 25 oscillations it was determined that for the sensor bar4 At 0.484 0.002 C. for A4; 2 1r. By using the literature value 7 (0.686t 0.013) X 10" (c.)-, (E. D. Pierron et al., Acta then,

Cryst. 21, 290, 1966), and the n, A, and I mentioned earlier, it isfound that 8=(l/n) (dn/dt)=(5.64i0.28) X 10" (c.)'1. 5 Most of theuncertainty in 8 results from the uncertainty in n.

As shown in FIG. 2, which is a plot of transmittance as a function oftemperature, the curves for both the measured and calculatedtransmissions vary from about percent maximum transmission to about 35percent minimum transmission (about 30 percent calculated) with atemperature change of approximately 0.25" C. The good fit of thecalculated and measured curves establishes the reliability and precisionof the method and the utility of measuring temperature changes withvariations in the transmission of energy through a body to provide aunique temperature sensor.

When the faces of the sensor material are antireflection coated withfilrrrs yielding 5.5 percent residual reflection, oscillations withtemperature with a peak to peak amplitude of 7.5 percent are observedwhen the sensor is accurately positioned.

It was also observed that when the sensor material is cooling it tendsto remain in the peak region of the transmission curve, while when thesensor is being heated it tends to dwell in the valleys. This selfregulation occurs because the absorption loss is proportional to theenergy passing through the sample. Consequently, a cooling GaAs sensorbar will cool slower and a heating bar will heat faster when more energypasses through it.

The good fit of the calculated and measured curves (FIG. 2) furtherindicate that the GaAs used was homogeneous and the faces flat andparallel to within dimensions which are small compared with the 10.6/3.43.l2p., wavelength of radiation. The experiment also illustrates theimportance of considering interference effects when using GaAs devices,e.g., as modulators in a C0 laser beam.

EXAMPLE 2 In FIG. 3 is illustrated an application of the utility of thetemperature sensor according to this invention, the working principlesof which were demonstrated in Example 1. In this embodiment, the sensormaterial 4, e.g., GaAs is situated at a distance remote from the energysource 1, e.g., a e0 laser as used in Example I. The temperature changeswithin the sensor material itself may be measured or, optionally, thetemperature variations of a body 22 in good thermal contact with thesensor may be measured. When a beam of coherent light B is directed fromlaser 1 through sensor 4, the output beam B is detected in detector 17.In some situations variations might occur in the incident beam B, eitherfrom the energy source or the medium through which the energy beampasses. In these situations, a reference beam B is directed from theenergy source by means of reflectors, e.g., mirrors 21 and 21a todetector 18 to determine the percent transmission level. Of course,reference beam B need not be used where there is no concern forvariations in the input energy or medium through which it passes.

The intensity of the energy transmitted through the sensor is measuredin detector 17 and the output signal from the detector is directed tosignal processor 19 where the output signal either is compared with thesignal from the reference beam and converts the resultant signal fromthe comparison to temperature values which are observed or recorded onreadout device 20, or when beam B is not used the output signal isconverted directly to temperature values. The temperature variationsdetected and measured by this system may be telemetered to monitoringstations remote from temperature event.

EXAMPLE 3 This Example will set forth alternative modifications of theforegoing embodiments for the purpose of increasing or decreasing thesensitivity of the temperature sensor to less than or greater than atemperature range of 025 C. as shown above.

One embodiment for increasing the temperature range over which amaximum-to-minimum transmission occurs is shown in FIG. 4. In FIG. 4 areshown several lengths of sensor material M, -M, transparent to a givenpart of the electromagnetic spectrum. For purposes of illustration thesensor material, GaAs of the same properties described in Example I, andradiant energy beams B 14 B of wavelength A, 10.6;1. are used. Theenergy source 23 can be either a single wide-aperture source, a singlenarrow-aperture source with scanning means for the various sensors M, Mor from multiple sources as in 23 a e. Similarly, the transmitted beamsb, B. can be detected by a single detector 24 which can monitor eachbeam sequentially or by a series of properly positioned detectors 24 a14 e. Beam B does not pass through the sensor material and is used todetermine the 100 percent transmission level. As in the precedingexamples, sensor material M, is 5.09 cm long; beam B, passing throughsensor M, will vary from maximumto-minimum transmission over-atemperature range of 025 C; beam B passing through 1.25 cm of sensormaterial M, will show a variation with a temperature change of 1 C.Similarly, beams B and B, passing through sensor materials M and M, of0.125 and 0.0125 cm lengths, respectively, will vary frommaximum-to-minimum transmission over ranges of and 100 C., respectively.

In contrast to increasing the temperature sensitivity range as describedabove one can decrease the temperature sensitivity range by increasingthe length of the sensor material to a length corresponding to amaximum-to-minimum transmission variation for the desired temperaturesensitivity range.

Another method for increasing or decreasing the temperature sensitivityrange involves the use of materials M, M (FIG. 4) which differ not onlyin length, but also in elemental composition having a different index ofrefraction n from that of the GaAs exemplified above.

In still another embodiment for changing the temperature sensitivityrange, reference is made to Equation (3) above; from this equation:

4Ln v+a (6) where Am is the change in temperature required to produce amaximum-to-minimum change in transmittance. Thus, for large A Am islarge. Conversely, when A, is small, Am is small and amaximum-to-minimum change in transmittance is produced with smallchanges in temperature. Hence, by varying the frequency A, of the sourceenergy one can increase or decrease the temperature sensitivity ofthesensor material.

EXAMPLE 4 Another embodiment of this invention makes use of thevariation in the reflection coefi'rcient of the sensor material when theindex of refraction varies. The reflection coeflicient of the material,under the same conditions noted above, i.e., monochromatic, coherentelectromagnetic energy and sensor material transparent thereto andhaving flat and parallel polished surfaces, will also vary cyclicallywith temperature, with the same temperature period as in the case oftransmission described above.

In FIG. 5 is shown a configuration in which the incoming beam B, fromenergy source 25 is reflected from the front and back surfaces of sensormaterial 26 which may have a highly reflective coating 27; the intensityof the reflected beam B, is detected and measured in detector 28. Theintensity of the reflected beam changes in a cyclic manner as describedin the transmission embodiment above. In FIG. 6 is shown a schematicdiagram of an application embodiment of the system configuration shownin FIG. 5. In this embodiment, the sensor material 26, e.g., GaAs asabove, is situated in good thermal contact with a body 22, e.g., asatellite, remote from the energy source 25 and/or detectors i7 and 18.As described above in the transmission case, variations in the inputenergy or in the medium through which it passes may render it necessaryor desirable to make use of a reference beam 13,, which is reflectedfrom reflector 29 attached to body 22 and received by detector 18. Whena beam B, of energy, e.g., coherent laser radiation, is directed to thesensor material it is reflected from the front and back surfaces of thesensor and the reflected beam B, is received by detector 17. Theintensity of the reflected beams is measured in the detectors from whichthe output signal is directed to signal processor 19 which converts theintensity signal to temperature values which are relayed to readoutdevice 20. As the temperature of the body, and, hence, of the sensorvaries, the intensity of the reflected beam changes in a cyclic manneras in the case of transmission described above.

In the reflection embodiment, as in the transmission embodiment, thesensitivity of temperature changes can be altered by changes in thesensor material, the length of the sensor material, the wavelength ofradiant energy or a combination of these.

While the foregoing embodiments described the use of GaAs as the sensormaterial, it will be appreciated that any material transparent to agiven part of the electromagnetic spectrum can be used herein. Thematerial selected should be such that the variation in its absorptioncoefficient a with temperature is small in the temperature rangeconcerned and a itself be only on the order of a few percent per cm. Onemay readily ascertain suitable crystal transparencies by reference tostandard tables of optical and electrical properties which listtransmission and absorption data for crystalline and amorphousmaterials. By way of illustration only, suitable materials include thealkali halides, e.g., NaCl and KC], cadmium sulfide and cadrrriumtelluride, Ge and various glasses.

Various other modifications may be made in the invention withoutdeparting from the spirit or scope thereof.

I claim:

1. Temperature sensing systems comprising in combination:

a. at least one body of single-crystal, semi-insulating gallium arsenidetransparent to a given part of the electromagnetic spectrum and whoselength and index of refraction change with a change in temperature;

b. means for irradiating said body with electromagnetic energy atfrequencies in the range in which said body is transparent;

0. means for detecting and measuring the intensity of that part of saidenergy not absorbed by said body; and

d. means for indicating the intensity output signal from said detectingand measuring means in temperature values.

2. Temperature sensing systems according to claim 1 wherein said body ofmaterial is in good thermal contact with a second body the temperaturevariation of which is to be monitored.

3. Temperature sensing systems according to claim 1 wherein thesensitivity of said body to temperature variations is determinable inaccordance with variations of parameters on the right-hand side of theequation:

M t, A (1 6) wherein Am equals the change required to produce a changefrom maximum to minimum output energy; A, is the wavelength of inputenergy; L is the length of said body, 7 is the expansion coefficient and8 is the temperature coefiicient of the index of refraction.

4. Temperature sensing systems according to claim 3 wherein a pluralityof bodies of material transparent to a given part of the electromagneticspectrum are used.

5. Temperature sensing systems according to claim 4 wherein at least oneof said bodies is a material difierent from other of said bodies and thesensitivity of each of said bodies controllable by varying any of thesaid parameters in the equation.

k i l

1. Temperature sensing systems comprising in combination: a. at leastone body of single-crystal, semi-insulating gallium arsenide transparentto a given part of the electromagnetic spectrum and whose length andindex of refraction change with a change in temperature; b. means forirradiating said body with electromagnetic energy at frequencies in therange in which said body is transparent; c. means for detecting andmeasuring the intensity of that part of said energy not absorbed by saidbody; and d. means for indicating the intensity output signal from saiddetecting and measuring means in temperature values.
 2. Temperaturesensing systems according to claim 1 wherein said body of material is ingood thermal contact with a second body the temperature variation ofwhich is to be monitored.
 3. Temperature sensing systems according toclaim 1 wherein the sensitivity of said body to temperature variationsis determinable in accordance with variations of parameters on theright-hand side of the equation: wherein Delta t pi equals the changerequired to produce a change from maximum to minimum output energy;lambda o is the wavelength of input energy; L is the length of saidbody, gamma is the expansion coefficient and delta is the temperaturecoefficient of the index of refraction.
 4. Temperature sensing systemsaccording to claim 3 wherein a plurality of bodies of materialtransparent to a given part of the electromagnetic spectrum are used. 5.Temperature sensing systems according to claim 4 wherein at least one ofsaid bodies is a material different from other of said bodies and thesensitivity of each of said bodies controllable by varying any of thesaid parameters in the equation.